Hydroelectric turbines are recognised as a means for effectively harnessing a clean and renewable energy source. Groups of hydroelectric turbines, installed in the sea, exploit natural currents caused by tides or by river flows near estuaries, to thereby generate electrical power for provision to utility grids, generally provided on shore.
Access to turbines installed in the sea is expensive and hazardous. Therefore, it is preferable, where possible, to avoid the use of components that present any risk of failure or wear. Thus, fixed pitch turbine blades are preferable to blades with adjustable pitch mechanisms, low-speed generators coupled directly to the turbine are preferable to high-speed generators coupled through gears and permanent-magnet excitation of the generators is preferred to arrangements that require brushes and slip rings or commutators.
In most cases, the groups of turbines comprise large numbers of turbines. Given the large number of turbines involved, it would be impractical and uneconomic to deliver the power to shore by a separate power cable connected to each turbine. Therefore, each power cable installed for transmitting power to shore is preferably arranged to collect the power from several turbines. In order for a significant amount of power to be transmitted from the group of turbines to a grid connection point on shore, which may be typically several kilometers from the turbines, the power cable operates at a high voltage. However, electrical elements within the turbine, such as generator windings, are normally designed to operate at much lower voltages for reliability and economy.
Furthermore, turbulence, and irregular topography of the seabed and nearby shorelines cause differences in the water flow velocity, and therefore, differences in the available power at each turbine within a group of turbines served by a single cable. In order to obtain the maximum power from the water current, a rotational speed of each turbine of the group of turbines should be adjusted in accordance with the prevailing water flow velocity. Thus, the turbines within a group are typically operated at different speeds and if fitted with permanent magnet generators, produce electrical outputs of differing frequency and voltage.
It is therefore an object of the present invention to provide a system for converting electrical power produced by a turbine to a form compatible with a power transmission system for transmitting the electrical power to shore, whilst ensuring the performance of the turbine and of the group of turbines as a whole, is optimised.